Vaccines to protect against viral infections have been used effectively to reduce incidence of human or animal viral diseases. One of the most successful technologies for viral vaccines is to immunize animals or humans with a weakened or attenuated strain of the target virus (a “live, attenuated virus”). Due to limited replication after immunization of a subject, an attenuated strain does not cause viral-related diseases because it can't be duplicated and passed onto a host. However, the attenuated virus is still capable of expressing viral antigens and can generate potent and long-lasting immune responses to the virus in the exposed subject. Therefore, upon subsequent exposure to the virus, the immunized individual has a reduced chance of developing diseases related to viral infection.
In order for live, attenuated viruses to be effective in a vaccine, the live, attenuated viruses must be capable of replicating after immunization and the viruses themselves must be protected from degradation in order to be used effectively and accurately. Some vaccines are sensitive to temperature extremes; either excessive heat or accidental freezing can inactivate the viruses. Maintaining an effective and protective environment throughout distribution is particularly difficult in developing countries or when treating patients in remote areas.